Washington: US should place conditions on release of about USD 3.5 billion aid to Pakistan based on "what we expect them to do", Defence Secretary Leon Panetta has told the Congress.

Short of recommending stopping of aid to Pakistan, Panetta, in a testimony to a Senate's appropriations panel said US would have to be careful about shutting the aid down.

Instead, he said he favoured placing conditions on aid based on "what we expect them to do". His testimony is crucial as influential lawmakers infuriated by Islamabad's closure of supply routes to NATO forces in Afghanistan and stalling of a crackdown on Haqqani network are pressing for blockage of aid to the country.

The Obama Administration has requested Congress for about USD 3.5 billion aid in military and economic assistance for Pakistan for fiscal 2013, Washington Post reported.

Panatta said that Pakistan's seven-month-long refusal to allow US and NATO supplies to cross its territory into Afghanistan is costing America an additional USD 100 million a month to fund alternative routes.

The Defence Secretary's testimony was the first time the Obama Administration has put a dollar figure on the extra amount.

Pakistan closed its border to NATO transit in November, after a US cross-border air assault inadvertently killed 24 Pakistani soldiers.

Washington Post said that the Pentagon and the Pakistani negotiators have agreed on a new payment structure for the transit, but the deal was stalled over Islamabad's demand for an apology for the killing of its soldiers.

The Obama administration has expressed "regret" and offered condolences, but it has said that an apology is unnecessary for an incident in which Pentagon investigators found fault on both sides.

The apology issue has become a political lightning rod in Pakistan, where anti-American sentiment runs high.

In the United States, Congress and the administration have grown increasingly irate over what is seen as Pakistani intransigence.

During a visit to the region last weekend, Panetta said the United States was "reaching the limits of patience" with Pakistan.

Until the border closure, Pakistan was the main transit route for the vast bulk of US and NATO supplies for the Afghanistan war, with goods arriving by ship at the Karachi port and trucked in cargo containers across the border.

Most of the goods now enter Afghanistan from the north, via Russia and Central Asia, at significantly higher expense.

The costs are expected to mount when US troop withdrawals are stepped up later this year.